1. Field of the Invention
The present invention relates to an image forming apparatus of a printer, a facsimile, a copier or the like for forming an image by using an electrophotography technology. Particularly, the invention relates to a charging technology for charging a photosensitive body thereof.
2. Related Art
Generally, an image forming apparatus using electrophotography technology includes a photosensitive body having a photosensitive layer at an outer peripheral face thereof, a charging unit for uniformly charging the outer peripheral face of the photosensitive body, an exposing unit for forming an electrostatic latent image by selectively exposing the outer peripheral face charged uniformly by the charging unit, a developing unit for constituting a visible image (toner image) by providing a toner which is a developing agent to the electrostatic latent image formed by the exposing unit and a transcribing unit for transcribing the toner image developed by the developing unit onto a record member of sheet or the like which is a transcribing object.
There is known a charging unit for charging the outer peripheral face of the photosensitive body utilizing a corona discharger referred to as a scorotron charger. The scorotron charger includes a discharge electrode, a supporting member for supporting the discharge electrode, a back plate for carrying out stable discharge and a grid for controlling charge potential on the photosensitive body. When charging is carried out, for example, by applying a voltage of −4KV through −6KV to the discharge electrode, applying −600V (potential dependent on potential intended to charge actually) to a grid and grounding the back plate or applying a potential the same as that of the grid to the back plate, corona discharge is generated from the discharge electrode and the photosensitive body can be charged to about −600V.
When the photosensitive body is charged by the above-described scorotron charger, as a parameter strongly effecting influence on the charge potential of the photosensitive body, there is a distance between the discharge electrode or the grid and a surface of the photosensitive body.
Therefore, the corona discharger of a related art is provided with a mechanism for making the distance between the discharge electrode and the surface of the photosensitive body constant (refer to, for example, JP-B2-2-10423, page 1, left column, page 2, right column, FIGS. 2 through 5; JP-Y2-2-3554, page 2, left column, FIGS. 2, 3; and JP-Y2-5-14282 page 2, left column, FIG. 5).
Further, there is also known a scorotron charger forming an aperture pattern of a grid by a small hole in a regular hexagonal shape to constitute an isotropic aperture rate with regard to a direction of moving the photosensitive body (for example, refer to JP-Y2-4-53650, page 1, left column, FIG. 3).
Further, there is known a corona discharger in which an inner face of a back plate is pasted with an insulator sheet such that a pasting rate at two end portions becomes larger than that of a central portion thereof to thereby prevent potential rise at the two end portions of the charger (for example, refer to JP-B2-6-58560, page 1, left column, FIG. 1)
The charge potential of the photosensitive body is inversely proportional to an electrostatic capacitance of the photosensitive layer. That is, the charge potential is proportional to a film thickness of the photosensitive layer and the thicker the photosensitive layer, the more increased is the potential.
Meanwhile, the film thickness of the photosensitive layer is provided with a film thickness deviation to some degree in view of fabrication. For example, an organic photosensitive layer which is generally used as a photosensitive layer is normally coated by dipping (dipping coating method). Although the dipping coating method is more excellent in film thickness stability than a ring coating method or the like, even with the dipping coating method, it is not rare to provide a film thickness deviation of 1 through 2 μm between an upper portion and a lower portion of dipping. Particularly, in the case of a large-sized printing photosensitive body having A3 size or more, the film thickness deviation becomes significant.
When charging is carried out for such a photosensitive body, even when the distance between the discharge electrode and the surface of the photosensitive body is maintained constant or the aperture pattern of the grid is formed to constitute the isotropic aperture rate with regard to the direction of moving the photosensitive body as in the above-described related arts, the charge potential on the photosensitive body is not constant. Further, even when the insulator sheet is pasted on the inner face of the back plate such that the pasting rate at the two end portions becomes larger than that of the central portion, the charge potential on the photosensitive body does not become constant.
For example, when there is a film thickness deviation of 1 through 2 μm between the upper portion and the lower portion of dipping of the photosensitive body, a difference of about 5 through 12 V is produced in the charge potential in an axis direction of the photosensitive body. This is the difference which cannot be disregarded in consideration of a request for high image quality formation in forming a color image of recent times. Normally, in order to achieve an excellent color image in an image forming apparatus, it is desired to make an in-plane dispersion of the charge potential (dispersion in the axis line direction of the photosensitive body) equal to or lower than 20 V, however, it is difficult to achieve the dispersion owing to an influence of a tolerance or the like of a part constituting the charger and it is a significant problem that the potential difference in the axis line direction of the photosensitive body becomes about 5 through 12 V from the start under such a situation.